Magnesium is also lost thru the urine, when using diuretics.

Diabetics or those with impaired glucose metabolism lose it in the urine. Coffee and alcohol also lower it. And yes, you should be concerned. Estimates are that about 70% of Americans in US are low in magnesium.

If you take a magnesium supplement with oxide in it, you will not absorb the magnesium. You need a chelated form instead.

My magnesium thread explains this:
http://neurotalk.psychcentral.com/thread1138.html

You can improve both with changes in diet.

This website shows you the content of foods:
http://nutritiondata.self.com/

Most foods with magnesium content, are whole foods..
example Edamame beans (very tasty and easy to prepare and inexpensive):
magnesium 99mg/serving
potassium 676 mg/serving.

Goal for daily consumption of potassium is 4,500mg
for magnesium 360mg elemental. My link explains "elemental".

A can of V8 vegetable juice= 800mg 12 oz potassium
low sodium is 1000mg!

You can use Nutrition data site to find foods high in these two minerals.

There are magnesium lotions available to use topically if oral is too laxative for you:
CVS epsom lotion (available at CVS stores)
Epsom-It

Being low in both chronically places you at risk for cardiac arrythmias, called QT events. Especially if you take any drugs that promote this effect.

http://www.azcert.org/

Magnesium is necessary for proper nerve functions, muscle relaxation, fat metabolism, and bone metabolism. Hundreds of processes in your body require it. Quality of life is lost and chronic illness occurs when magnesium is low. If you get too low in it your heart may suffer and you may even die as a result.

If you take a supplement orally start at 1/2 the RDA and see what happens. If you use the topical liberally you won't need oral at all.